1. Field of the Invention
The present invention relates to optical disk systems of the kind adapted for optically writing and reading high density information and more particularly to improvements in write/read apparatus, methods and disk unit constructions, which ameliorate adverse effects of recording debris.
2. Description of the Prior Art
One popular type of optical disk comprises a record layer of thermally ablatable material formed on a disk support. Information is recorded by modulating a tightly focused laser beam to form pits along tracks on such a disk while it is rotating. The pits can be extremely small (e.g. on the order of a micron) in size; therefore they are susceptible to damage by contact and obscurement by dust particles. To avoid these problems many disk unit constructions include a protective cover over the record layer.
U.S Pat. Nos. 4,074,282 and 4,365,258 disclose disk units wherein the protective cover is a disk-shaped cover sheet supported around its outer periphery in spaced relation to the disk record element so as to provide a sealed disk unit having an enclosed, dust-free air space above the record layer. Such sealed disk units offer advantages from several viewpoints; however, we have found that, in certain such disk unit embodiments, decreases in writing sensitivity and reading fidelity can occur because of debris from the ablative pit-forming events.
Specifically, we have found that in sealed disk units, certain dye/binder recording layers, e.g. some of the embodiments disclosed in U.S. Pat. Nos. 4,336,545 and 4,360,908, form ablative debris, which coagulates into particles as large as a micron in size. When such large debris particles deposit on a record portion of the disk unit they can cause a decrease in signal-to-noise ratio and a significant increase in the error rate.